Simulations and Diagnostics for the Front End Test Stand Simon Jolly Imperial College 18 th April 2007

18/4/072 From HPPA’s to FETS (D. Findlay) New generation of High Power Proton Accelerators (HPPAs) required for: –neutron spallation sources –neutrino factories –Transmutation facilities –Accelerator driven power reactor systems –Tritium production High power is difficult: –Imperative to keep beam losses low (~1 W/m) ISIS only ~0.2 MW, but ×2 beam losses would make life very difficult (2–3 mSv annual dose limit) Need good quality beam –Space charge issues significant Implies beam chopper necessary even if no rings involved –Need to control transients — RF and target issues Implies beam chopper very desirable “The Front End Test Stand (FETS) is intended to demonstrate the early stages of acceleration (0-3MeV) and beam chopping required for HPPA’s”

18/4/073 FETS Specification (A. Letchford) 60 mA H- ion source 65 keV 3 solenoid magnetic LEBT 324 MHz, 3 MeV RFQ High speed beam chopper & MEBT Conventional and non-destructive diagnostics Up to 2 ms pulse length Up to 50 pps rep. rate ‘Perfect’ chopping

18/4/074 FETS Layout FETS main components: High brightness H- ion source. Magnetic Low Energy Beam Transport (LEBT). High current/duty factor Radio Frequency Quadrupole (RFQ). Very high speed beam chopper. Comprehensive diagnostics. RFQ Chopper

18/4/075 LEBT Design Low Energy Beam Transport takes beam from ion source and focuses into RFQ. Design based on ISIS LEBT - three solenoids between drift areas. Optimise design using GPT simulations of beam envelope and profile along LEBT. 25 cm30 cm19 cm30 cm24 cm30 cm15 cm 0.21 T0.05 T0.25 T d1d1 d2d2 d3d3 d4d4 H–H– RFQ Solenoids Drift areas (vacuum) Constraints: B < 0.6 T, solenoids long enough to ensure flat axial field (d ≥ 25cm). d 1 = 25cm, d 4 = 15cm (minimum for vacuum equipment and diagnostics). Overall length must not be too long (cost). RFQ acceptance: 2-3mm, 50-60mrad (from ~20mm, with  x/y = 0.3  mm-mrad).

18/4/076 LEBT Solenoid Focussing “Hard” focussing leads to large emittance growth. “Soft” focussing, with 2 strong and 1 weak solenoid, give better results. Hard focussing Soft focussing

18/4/077 LEBT Solenoid Focussing (2) Solenoid focussing solutions very sensitive to input conditions: very little data to go on! “Optimised” LEBT produces very different results when using real ion source measurements. Initially only 2 sources of data for GPT beam conditions (which look very different…): –Ion source emittance measurements. –MAFIA simulations of ion source output. No information on X-Y profile or space charge…

18/4/078 LEBT Simulation: Trajectories Beam Z-X trajectories using optimised Weak Focussing Solution, but measured parameters: beam only shrinks from R max =25mm to R max =15mm…

18/4/079 Ion Source Emittance Data X Y X Y Measured ion source emittance data gives emittance ~600mm from ion source exit: –  Hrms = 0.92,  Vrms = 1.01  mm mrad. –x rms = 26.0 mm, x’ rms = 32.0 mrad. –y rms = 24.6 mm, y’ rms = 35.0 mrad. MAFIA simulations give emittance at exit of ion source cold box. Using GPT to try and match one to the other totally hopeless…

18/4/0710 Space Charge Simulations in GPT Treat set as mono-energetic 2D slice at 600mm, input to GPT and track backwards using 2D space charge model and levels of space charge compensation. Try to match X-Y profile at 0mm to real exit aperture of cold box and results from MAFIA simulations, using different space charge compensation and time-reversed simulation. Various Space Charge models tested for consistency (2D and 3D).

18/4/0711 Results: Input Data (Emit) Emittance plots for “initial” beam data XY

18/4/0712 Results: Emittance, 10% SC Emittance plots for beam at 0mm, 10% space charge XY

18/4/0713 Results: Emittance, 30% SC Emittance plots for beam at 0mm, 30% space charge XY

18/4/0714 Results: Emittance, 50% SC Emittance plots for beam at 0mm, 50% space charge XY

18/4/0715 The Pepperpot Emittance Scanner Current Allison-type scanners give high resolution emittance measurements, but at fixed z-position and too far from ion source. X and Y emittance also uncorrelated, with no idea of x-y profile. Correlated, 4-D profile (x, y, x’, y’) required for accurate simulations. Pepperpot reduces resolution to make correlated 4-D measurement. Moving stage allows measurement at different z- locations: space charge information. Added bonus: make high resolution x-y profile measurements

18/4/0716 Pepperpot Principle H - Ion Beam Tungsten screen Copper block Quartz screen H - Beamlets Fast CCD Camera Beam segmented by tungsten screen. Beamlets drift ~10mm before producing image on quartz screen. Copper block prevents beamlets from overlapping and provides cooling. CCD camera records image of light spots. Calculate emittance from spot distribution.

18/4/0717 Ion Source Development Rig Ion source test facility vacuum tank Ion source Emittance scanners

18/4/0718 Vacuum bellows Camera Moving rod Shutter Mounting flange Pepperpot head Bellows Tungsten mesh Beam profile head Mk.II Pepperpot Design

18/4/0719 Pepperpot Results Raw data

18/4/0720 Pepperpot Emittance Plots

18/4/0721 Scintillator Measurements 5 kV Ext5.5 kV Ext6 kV Ext6.5 kV Ext 7 kV Ext8 kV Ext9 kV Ext11 kV Ext

18/4/0722 Conclusions Particle dynamics simulations extremely sensitive to input conditions. Pepperpot finally providing necessary information for 4-D emittance profiles. Significant aid to ion source development: –Profile measurements. –Multiple emittance measurements. More results in time for DIPAC’07…

18/4/0723 Relevant Experience Experience with numerous accelerator simulation codes: MAD, DIMAD, LIAR, MatLIAR, Guinea-Pig, GPT (particle dynamics and space charge). Practical experience with both electron and hadron machines. Project management for Hilger Crystals: novel x- ray system for afterglow measurement. Previous work in Medical Physics (Whittington Hospital).

18/4/0724 Spare Slides

18/4/ mm 35kV 17kV Platform Ground Platform DC Power Supply Pulsed Extract Power Supply Post Extraction Acceleration Gap Laboratory Ground Extraction Electrode, Coldbox and Analysing Magnet all Pulsed 35keV H - Beam kV

18/4/0726 Scintillator Problems Pepperpot rapidly became “scintillator destruction rig”. Scintillator requirements: –Fast (down to 500ns exposure). –High light output. –Survives beam (<1 micron stopping distance). High energy density from Bragg peak causes severe damage… Finally settled on Ce- doped quartz.

18/4/0727 YAG:Ce Spot Intensity Ruby (500ms exposure) P43 (10ms exposure) P46 (500ms exposure) YAG:Ce (100ms exposure)

18/4/0728 Simulated Beam Profile Camera res. = 49 microns/pixel Angle res. = 4.88 mrad Image width: 100mm

18/4/0729 Simulated Beam: Emittance Plots  x = 6.01   y = 6.51  x-x’ y-y’